01 -
Preheat oven to 375°F. Butter an 8-cup ovenproof pie or casserole dish.
02 -
Melt butter in a large saucepan over medium heat. Add onions and garlic; cook for 6 to 8 minutes until softened and translucent without browning. Remove from heat.
03 -
In a bowl, whisk together heavy cream, whole milk, Dijon mustard, freshly grated nutmeg, salt, and black pepper until fully combined.
04 -
Arrange half of the potato slices in the prepared dish with slight overlap. Evenly spread half of the onion and garlic mixture over potatoes. Sprinkle half of the grated Gruyère and cheddar cheeses on top.
05 -
Repeat the layering process with remaining potatoes, onion mixture, and cheeses to form a second complete layer.
06 -
Pour the cream mixture evenly over all layers. Gently use a spatula to press down and compact the layers for even cooking.
07 -
Cover the dish with aluminum foil and bake for 40 minutes at 375°F.
08 -
Remove the foil and continue baking for an additional 20 minutes until the top is golden brown and the filling bubbles at the edges.
09 -
Allow the dish to rest for 10 minutes before serving. Garnish with finely chopped fresh chives if desired.